There is known a catheter employed to be inserted into a biological lumen such as a blood vessel and to diagnose the inside of the biological lumen by ultrasonic waves (for example, refer to Japanese Patent No. 3954888).
Japanese Patent No. 3954888 discloses a catheter including a catheter main body that has a lumen, and a shaft that is inserted into the lumen of the catheter main body and has an ultrasonic wave oscillation portion emitting the ultrasonic waves at a distal portion. The catheter rotates the shaft inserted into the lumen of the catheter main body around an axis of the shaft and moves the shaft in a proximal direction, and thus, it is possible to acquire an ultrasonic wave image of a vascular wall.
In such a catheter, in order to improve operability and prevent kinks, as a medical tube employed for the catheter main body, a tube of which rigidity is gradually decreased from its intermediate portion toward a distal side is required.